This invention relates to the low-temperature distillation of air to obtain low-purity oxygen and nitrogen-rich products. The term "low-purity oxygen" as used throughout the present specification and claims is intended to mean a product having an oxygen content of less than 99.5 mole percent.
It is believed that very large quantities of low purity oxygen will be required by processes now being developed for converting coal to liquid or gaseous products. Another use for low purity oxygen is in a process for converting refuse to useful gaseous products as described in Anderson U.S. Pat. No. 3,729,298. Hence, a process for producing low-purity oxygen in large quantities at low cost is desirable.
A common system for low temperature fractionation employs a higher pressure distillation column having its upper end in heat exchange relation with the lower end of a lower pressure distillation column. Cold compressed air is separated into oxygen-enriched and nitrogen-rich liquids in the higher-pressure column and these liquids are transferred to the lower-pressure column for separation into nitrogen- and oxygen-rich products. Examples of this double-distillation column system appear in Ruheman's "The Separation of Gases," Oxford University Press, 1945.
Large quantities of energy are required to compress the feed air for such a process. Hence, in these times of rising energy costs, a saving of energy is important. Another problem associated with conventional systems is the large diameter of the lower pressure column, which must handle substantially all of the air fed to the system at relatively low pressure. One way to reduce the energy cost of the low-temperature distillation of air, as disclosed by Potts in U.S. Pat. No. 3,066,494, is by a dual-feed-pressure process. Such a process compresses only part of the feed air to the operating pressure of the higher pressure distillation column. The remainder of the feed air is compressed to a lower pressure and fed to the lower pressure column. The difficulty with the Potts process is that the maximum oxygen purity attainable is limited to about 90 mole percent. This limitation results from the low-pressure feed stream's by-passing the higher pressure column and entering the lower pressure column without having had the benefit of a prior separation. Furthermore, use of the Potts process does not achieve a reduction in the diameter of the lower pressure distillation column.
Schlitt, in U.S. Pat. No. 2,209,748 discloses a dual-feed-pressure process that uses an auxiliary column to remove a portion of the nitrogen from the low-pressure feed stream prior to feeding the low-pressure stream into the lower pressure column. Schlitt eliminates the higher pressure column. Schlitt's process is able to achieve purities in excess of 90 mole percent at reduced, but still relatively high energy costs. However, the present invention is able to achieve energy usage even lower than that of Schlitt.